The invention is based on a switched-mode power supply which contains a switching transistor as well as an isolating transformer having a primary winding and a secondary winding. These require a relatively high level of component complexity, particularly if regulation information must be transmitted from the secondary side to the primary side in order to stabilize an operating voltage on the secondary side. Various embodiments are known for transmitting this information, for example using an optocoupler. DE 40 04 707 discloses a circuit in which a regulating signal is transmitted back via a transistor stage in a brief time interval during a quiescent phase of the isolating transformer. The publication "UNIQUE CHIP PAIR SIMPLIFIES ISOLATED HIGH SIDE SWITCH DRIVE" discloses an smps having the features of the preamble of claim 1.
The object of the present invention is to reduce the component complexity for switched-mode power supplies of the type.
The invention is as set out in Claim 1. Advantageous developments of the invention are specified in the subclaims.
The stabilized operating voltage, which is preferably used for supplying a load having a low power consumption, contains a series-connected light-emitting diode to indicate operation of relevant equipment. Since the light-emitting diode is connected downstream from the rectifier diode and upstream of a stabilization capacitor, it is operated at the pulsating switching frequency of the switched-mode power supply. At the high switching frequencies used here, the light-emitting diode has a very low power consumption of only, for example, 3 milliwatts; there is therefore no need for any resistor in series with the light-emitting diode.
According to the invention, the control input of the switching transistor is driven by the driver stage via a differentiating element, the switching transistor being arranged between the operating voltage on the input side and the primary winding of the isolating transformer. Since this results in the control input being DC-decoupled, it can be connected via a resistor to the potential of the operating voltage on the input side.
The driver stage produces square-wave pulses for driving the switching transistor. The time constant of the differentiating element is chosen such that the switching transistor is switched off by a positive pulse flank and is switched on by a negative pulse flank. The output voltage of the switched-mode power supply can be regulated by varying the pulse width and/or the frequency.
Since the switching transistor is connected upstream of the primary winding of the isolating transformer in the current flow direction, that end of the primary winding which is at the lower potential is used to produce a supply voltage on the primary side. This can be stabilized by means of a network which contains two zener diodes and at the same time provides the starting current for the driver stage.
The switched-mode power supply may have a very compact construction and, because of its low power consumption, is suitable in particular for use as a separate standby power supply for a television set or video recorder. In the standby mode, it drives, for example, only an infrared receiver and, for this purpose, requires a power consumption of less than 100 mW, including loads.
The invention will be explained in the following text using, by way of example, a schematic drawing in which the figure shows a circuit diagram of a switched-mode power supply designed according to the invention.
The switched-mode power supply in the figure contains a bridge rectifier having four diodes DP01 - DP04 for producing an operating voltage UB1 on the input side from an AC voltage VAC. The operating voltage UB1 is stabilized by two series-connected zener diodes DP05 and DP06 in parallel with two series-connected capacitors CP05 and CP06. A supply voltage UB2 is at the same time stabilized via the mutually connected centre tap of these series circuits. Two reactive elements C1 and C2 considerably reduce the AC voltage applied to the bridge rectifier. In this exemplary embodiment, the operating voltage UB1 is 36 volts, and the supply voltage UB2 is 6 volts.
The operating voltage UB1 is connected to the emitter of a switching transistor TP21 whose collector is connected to the primary winding W1 of an isolating transformer Tr1. The second connection 4 of the primary winding W1, which is at a lower potential, is connected to the supply voltage UB2. The supply voltage UB2 is thus not only provided by the zener diode DP02 but is also obtained from the primary winding W1 by operation of the switching transistor TP21 and is stabilized by the zener diode DP06 and the capacitor CP06. The supply voltage UB2 is used to supply a driver stage IP01 and a transistor stage TP30.
In this exemplary embodiment, the driver stage IP01 is an oscillator in the form of an appropriately connected operational amplifier. It operates as an astable multivibrator and produces square-wave signals. The output of the driver stage IP01 is connected via a differentiating element to the control input of the switching transistor TP21. In this exemplary embodiment, the differentiating element comprises a series circuit formed by a resistor RP20 and a capacitor CP20, whose values are chosen such that they differentiate the switching flanks of the oscillator signal.
Connected in parallel with the emitter and the base of the switching transistor TP21 there is a resistor RP21 which is used to draw the base potential of said switching transistor TP21 to the operating voltage UP1 so that the switching transistor is in the switched-off state when there is no drive signal. Differentiation of the square-wave pulses results in suitable positive and negative pulses being produced alternately in order to switch the switching transistor TP21 on and off.
The isolating transformer TR1 contains a primary auxiliary winding W3 which is used to provide a regulating signal for stabilizing one or more secondary output voltages. This regulating signal is amplified in a transistor stage TP30 and is supplied to the oscillator IP01 in order to regulate the pulse width and/or the frequency.
The switched-mode power supply operates, for example, at an oscillator frequency of 100 kHz, which is governed by the oscillator IP01. Alternatively, the switched-mode power supply can be synchronized, for example over a frequency range of 40 - 150 kHz. With appropriate component values, the differentiating element can be matched to the desired oscillation frequency or an oscillation range.
The following values were used for relevant components for the drive level illustrated in the figure:
The isolating transformer Tr1 of the switched-mode power supply contains one or possibly more secondary windings W2 for producing supply voltages. In this exemplary embodiment, a relatively small load IE, for example an infrared receiver of a television set or of a video recorder, is operated by means of a supply voltage US1.
The exemplary embodiment in the figure is preferably used to produce low power levels on the secondary side and has very high efficiency. The transformer Tr1 may have a very compact design, particularly because it is designed only for a low operating voltage. The switched-mode power supply is therefore particularly suitable for use as a separate standby switched-mode power supply for entertainment electronic equipment. Alternatively, it can be designed for higher power levels, and is thus also suitable, for example, for use as a DC-DC converter in conjunction with a car battery. The switched-mode power supply is started in a simple manner via the zener diode DP05.